You get this achievement in the second room towards the end. Once in the second room, follow the following directions:
1) Get the valve from your inventory. You should have picked the valve up in the first room before leaving it.
2) Combine the valve with the wall tube.
3) Open the tap to let the water run (not the valve, THE TAP)
4) Pick up the hosepipe lying next to the tap.
5) Combine the hosepipe with the tap.
6) Pick up the hosepipe and aim it at the fire (without moving too far away from tap). Water should be flowing at the end of the hosepipe.
7) Wait for the fire to be extinguished, and fight with the monster. Attack it by looking at it and pressing
This achievement should pop after you kill the monster:
8) Pick up the bucket and drop it (without overturning it) near the hosepipe.
9) Pick up the hosepipe and aim it at the bucket to fill it up with water.
11) Pick up the bucket full of water once your guy say something along the lines of "I think that should be enough."12) Drop it (without overturning it) under the two exposed cables at the other side of the room. The achievement will pop at this time.
To finish the room and advance to the next one simply push on the button at the door and proceed to the next room.